1. Field of the Invention
The invention relates to the therapeutic or prophylactic treatment of fungal infections.
2. Summary of the Related Art
Fungal infections have become a serious health concern, especially in immunocompromised patients. Ha and White, Antimicrobial Agents and Chemotherapy 43: 763-768(1999) teaches that Candidiasis, which is caused by the pathogenic yeast Candida albicans, is the most frequent fungal infection associated with AIDS and other immunocompromised states. Weig et al., Trends in Microbiology 6: 468-470(1998) discloses that the frequency of Candida infections has increased in recent years and has been accompanied by a significant rise in morbidity and mortality. Many of these infections take place in the hospital setting. Baillie and Douglas, Methods in Enzymology 310: 644-656(1999) teaches that a majority of nosocomial septicemias caused by Candida species derive from biofilm formation on catheters and shunts.
Recently, there has been great interest in identifying genes that may be implicated as important virulence factors in these infections. Calera et al, Infection and Immunity 68: 518-525(2000) discloses that the SSK1 response regulator gene from C. albicans is essential for normal hyphal development and virulence. Alex et al., Proc. Natl. Acad. Sci. USA 95: 7069-7073(1998) teaches that COS1, a two-component histidine kinase, is required for normal hyphal growth of C. albicans, and may play a role in virulence properties of the organism. Alonso-Monge et al., J. Bacteriology 181: 3058-3068(1999) teaches that deletion of the C. albicans gene encoding the mitogen-activated protein kinase HOG1 causes derepression in serum induced hyphal formation and a dramatic increase in the survival time of systemically infected mice. Csank et al., Infection and Immunity 66: 2713-2721(1998) discloses that disruption of the C. albicans mitogen activated protein kinase CEK1 adversely affects the growth of serum induced mycelial colonies and attenuates virulence in a mouse model for systemic candidiasis. These and other studies have suggested that hyphal growth may be an important virulence factor in C. albicans. Lo et al., Cell 90: 939-949 (1997) teaches that nonfilamentous C. albicans mutants are avirulent. However, all of these mutants have other phenotypic changes, and at least some of them retain some virulence. Thus, it is not entirely clear that the virulence is being attenuated solely through blocking of germ tube formation.
The exact mechanism by which hyphal growth acts as a virulence factor is also not known with certainty. Kretschmar et al., Infection and Immunity 67: 6637-6642 (1999) teaches that there is a correlation between germ tube length and organ invasion in C. albicans clinical isolates. Van't Wout et al., J. Antimicrob. Chemother. 25: 803-811 (1990) discloses that C. albicans may resist intracellular killing by macrophages through the formation of germ tubes.
A variety of antifungal antibiotics have been developed, some of which also affect hyphal growth. Bremm et al., Candida and Candidamycosis (E. Tumbay, Ed.), Plenum Press, New York (1991) teaches that azole compounds have an influence on fungal adhesion. Brenciaglia et al., Chemioterapia 5: 200-203 (1986) teaches that amphotericin B interferes with C. albicans adherence in vitro at sub-minimum inhibitory concentrations (MIC), but that 5-fluorocytosine, nystatin, miconazole and ketoconazole interfere with adherence only at concentrations above MIC values. Amphotericin B and the azoles have become a first-line treatment for Candida infections. Martin, J. Antimicrob. Chemother 44: 429-437 (1999) teaches that fluconazole is preferably administered at 300-400 mg/day, depending on the localization of the infection, and that higher doses can be used successfully.
There is a need for even less toxic treatment regimens than those presently available. Tamburini et al., U.S. Pat. No. 5,833,946 teaches that there is a need for effective anti-Candida agents having fewer toxicological problems than amphotericin B, and which by virtue of their lower toxicities can be administered to high risk patients either prophylactically or at the earliest signs of infection, without the need for a firm diagnosis. Philpott-Howard et al., J. Antimicrob. Chemother. 31: 973-984 (1993) discloses that over 5% of patients treated with fluconazole had adverse reactions, possibly related to the treatment, about half of which necessitated discontinuation of therapy.
For nearly two decades, scientists have talked about the possibility of using sub-lethal (to the fungus) concentrations of agents that inhibit hyphal growth as a therapeutic treatment for fungal infections. Plempel and Berg, Dermatologica 169: 11-18 (1984) discloses that when C. albicans cells are pretreated with sub-inhibitory concentrations of azole compounds and injected intravenously in mice, the resultant infections are of reduced severity in comparison to controls. Haller, Am. J. Obstet. Gynecol. 152: 939-944 (1985) discloses that azole compounds inhibit mycelial growth in vitro at even 1% of MIC. Nugent and Couchot, J. Infect. Dis. 154: teaches that sub-lethal doses of amphotericin B inhibit C. albicans germ tube formation and adherence in vitro. Mehentee and Hay, J. Antimicrob. Chemother. 25: 111-119 (1990) discloses that sub-inhibitory concentrations of amphotericin B, ketoconazole and itraconazole significantly reduce the adherence of C. albicans to murine gastrointestinal mucosal surfaces in vitro. Ghannoum et al., Antimicrob. Agents Chemother. 36: 2239-2244 (1992) teaches that subinhibitory concentrations of fluconazole and amphotericin B block C. albicans ability to adhere to and injure human umbilical vein endothelial cells in vitro. Braga et al., Arzneimittelforschung 42: 1368-1371 (1992) teaches that sub-inhibitory concentrations of ciclopirox causes a significant reduction in C. albicans adherence to human buccal cells and human vaginal cells in vitro. Braga et al., Chemotherapy 42: 259-265 (1996) discloses that sub-inhibitory concentrations of rilopirox, nystatin and fluconazole interfere with C. albicans adherence to human vaginal cells in vitro. Ellepola and Samaranayake, Arch. Oral. Biol. 43: 999-1007 (1998) suggests that sub-therapeutic doses of antifungals may modulate oral candidal colonization, based upon in vitro studies. Ellepola and Samaranayake, J. Oral Pathol. Med. 27: 325-332 (1998) teaches that sub-lethal concentrations of nystatin, 5-fluorocytosine, ketoconazole and fluconazole reduces binding of C. albicans to buccal epithelial cells in vitro. Ellepola and Samaranayake, J. Oral Pathol. Med. 27: 213-219 (1998) discloses that sub-lethal concentrations of nystatin, amphotericin B, 5-fluorocytosine, ketoconazole and fluconazole inhibit germ tube formation by C. albicans in vitro. Ha and White, Antimicrobial Agents and Chemotherapy 43: 763-768 (1999) teaches that as little as 0.1 μg/ml fluconazole reduces hyphal formation in a susceptible isolate of C. albicans in vitro.
Recently, considerable doubt has been cast as to whether fungistatic agents, such as the azoles, can actually inhibit morphogenic transformation at sub-MIC levels. Hawser et al., J. Antimicrob. Chemother. 38: 579-587 (1996) teaches that when NCCLS guidelines are used in the study, ketoconazole can only block the transformation at very high concentrations. Hawser and Islam, J. Antimicrob. Chemother. 43: 411-413 (1999) teaches that antifungal agents that are more fungicidal, such as amphotericin B and the candins are able to inhibit morphogenic transformation at sub-MIC levels, whereas fungistatic agents, such as the azoles, are not.
Despite all of these in vitro studies over many years, no one has succeeded at therapeutically or prophylactically treating fungal infections using sub-lethal concentrations of an anti-invasin compound. There is, therefore, a need for new methods for therapeutically or prophylactically treating fungal infections using sub-lethal concentrations of an anti-invasin compound.